Total internal reflection (TIR) is an optical event where light cannot pass through a boundary and is totally reflected. For TIR to occur, light strikes a boundary between two media at an angle larger than a critical angle, where the critical angle is a function of the refractive indices of the two media. The critical angle is measured with respect to the normal to the boundary.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.